1. Field of the Invention
The present invention relates to a fuel cell system in which electrical energy of a fuel cell is discharged when power generation in the fuel cell system is stopped, and relates to a method of controlling discharge electrical energy in the fuel cell system.
2. Description of the Related Art
For example, a polymer electrolyte fuel cell employs a membrane electrode assembly which includes an anode (fuel electrode), a cathode (air electrode), and a polymer electrolyte membrane interposed between the electrodes. The electrolyte membrane is an ion exchange membrane. The membrane electrode assembly is sandwiched between a pair of separators. A fuel gas flow field is formed between the anode and one of the separators, and an oxygen-containing gas flow field is formed between the cathode and the other of the separators. In use, normally, a predetermined numbers of the membrane electrode assemblies and separators are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas flow field. The fuel gas flows through the fuel gas flow field along the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the suitably humidified electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating DC electrical energy. Further, in the fuel cell, an oxygen-containing gas such as the air is supplied to the oxygen-containing gas flow field, and the oxygen-containing gas flows along the cathode for reaction. At the cathode, hydrogen ions from the anode combine with the electrons and oxygen to produce water. If water at the cathode becomes excessive, water clogging may occur. Thus, in the fuel cell system, in order to eliminate water at the cathode, a technique of the scavenging process for the cathode (hereinafter simply referred to as the “cathode scavenging process” or the “scavenging process”) is performed. In the scavenging process, the oxygen-containing gas is continuously supplied to the cathode when the supply of the fuel gas to the fuel cell is stopped.
In the case where the fuel cell system is applied to a vehicle, according to one proposal, an energy storage (secondary battery) and the fuel cell are mounted in the vehicle in parallel for driving a motor. The technique is adopted, e.g., for achieving the desired responsiveness in the fuel cell system when the fuel cell system is operated variably in accordance with the driving power, for supplying electrical energy to auxiliary devices such as an air compressor of the fuel cell system at the time of starting operation of the fuel cell, and for charging the energy storage using regeneration energy of the motor at the time of deceleration of the vehicle to use the energy for assistance of next acceleration, thereby achieving improvement in the efficiency of the fuel cell vehicle.
When the fuel cell system needs to be stopped, in order to avoid corrosion of the fuel cell components or the like, the process of discharging electrical energy from the fuel cell is required for eliminating the voltage between the electrodes of the fuel cell. In a proposed technique, the discharged energy is utilized for charging an energy storage (see Japanese Laid-Open Patent Publication No. 2004-253220).
In the technique, the fuel cell and the energy storage are connected through a DC/DC converter. After the supply of reactant gases to the fuel cell is stopped, the reactant gases remaining in the fuel cell is used for charging the energy storage through the DC/DC converter. Then, if the voltage of the fuel cell exceeds the voltage of the energy storage, the DC/DC converter is operated in a step down mode. When the voltage of the fuel cell becomes the voltage of the energy storage or less, the DC/DC converter is operated in a step up mode.
However, in practice, at the time of discharging electrical energy from the fuel cell, components (electrical devices other than the energy storage) such as an electrical control unit (ECU), contactors, the DC/DC converter, and a downverter (down converter) need to be in operation. As in the case of the technique disclosed in Japanese Laid-Open Patent Publication No. 2004-253220, if all the electrical energy discharged from the fuel cell is charged in the energy storage, the other electrical devices temporarily obtain electrical energy from a low voltage battery of +12[V]. The temporal electrical energy needs to be provided to the low voltage battery by power generation of the fuel cell, or supplied from the energy storage to the low voltage battery, before or after the discharge process, or at the time of the next operation of the fuel cell. At this time, charge/discharge losses in the energy storage, and switching losses in the converters such as the DC/DC converter and the downverter occur disadvantageously.
The problem is shown schematically in a time chart of FIG. 9A. At the time t0, the supply of the fuel gas to the fuel cell is stopped. From this time t0, the cathode scavenging process is performed by driving the air compressor using electrical energy discharged from the energy storage. Thus, the electrical energy stored in the energy storage is decreased from the initial electrical energy P1 to the electrical energy P2 by energy consumption of the electrical energy P5 required for the cathode scavenging process (P1=P5+P2). At this time, the discharge losses in the energy storage and DC/DC converter losses P7 occur for the electrical energy P6 required for the air compressor to actually perform the cathode scavenging process (P5=P6+P7).
After the cathode scavenging process is finished, the electrical energy P8 discharged from the fuel cell in the discharge process is used for charging the energy storage through the DC/DC converter. By the charge, electrical energy in the energy storage is restored from P2 to P3 (P3=P2+P8−P9) at the time t2. However, at the time of the discharge process (charge process for the energy storage), charge losses in the energy storage and DC/DC converter losses P9 occur. At the time of the charging process, if the voltage of the fuel cell becomes lower than the voltage of the energy storage, the DC/DC converter is operated in the step up mode.
As described above, in the conventional discharge process, the charge/discharge losses due to the scavenging process at the time of stopping the supply of the fuel gas, and the charge/discharge losses due to the discharge process of the energy storage after the scavenging process occur disadvantageously. Further, the circuit scale for operating the DC/DC converter in the step up mode is large disadvantageously.